Electronic switching and mixing circuit for pulsating direct current



y 9, 1953 D. w. BLANCHER 7 ELECTRONIC SWITCHING AND MIXING CIRCUIT FOR PULSATING DIRECT CURRENT Filed Oct. 16. 1950 INVENToR. D. W. Bjancher ATTORNEY Patented May 19, 1953 ELECTRONIC SWITCHING AND MIXING CIR- CUIT FOR PULSATING DIRECT CURRENT Donald W. Blancher, North Hollywood, Calif., as-

signor to Bendix Aviation Corporation, South Bend, Ind., a corporation of Delaware Application October 16, 1950, Serial No. 190,429

3 Claims. (Cl. 250-27) This invention relates to electronic circuits for alternately blocking and passing direct currents at a .rapid rate without the use of mechanical switches.

An object of the invention is to provide a simple and reliable circuit without mechanical switches for alternately blocking and passing a direct current without appreciable distortion thereof.

Another object is to alternately pass short increments of two direct currents to a common circuit at a rapid rate whereby they can be impressed upon the vertical deflection plates of a single cathode ray oscilloscope in rapid succession to produce thereon two simultaneously visible curves showing the respective magnitudes of the two currents.

Other more specific objects and features of the invention will appear from the description to follow:

- A system in accordance with the invention for alternately blocking and passing a single unidirectional current includes a tube having a cathode, control grid and anode in which the cathode and anode 'are in series with the source of direct current to be switched, and the grid is alternately swung sufficiently negative to block conduction through the tube, and sufiiciently positive to produce linear conduction characteristics between the cathode and anode. The desired variable grid voltage may be produced by a square wave oscillator such as a multi-vibrator, and is applied through a condenser to the grid; a resistor network and the grid conductance being used to determine the positive and negative limits of grid voltage swing.

Two such circuits operating in phase opposition under control of the same multi-vibrator oscillator can be used to alternately apply short increments of two separate direct currents to a single circuit such as the vertical deflection plates of a. cathode ray oscilloscope.

A 'full understanding of the invention may be had from the following detailed description read in connection with the drawing, in which:

Fig. 1 is a schematic circuit diagram of a system in accordance with the invention.

Fig. 2 is a set of curves showing the switching potentials developed at certain points in the circuit of Fig. 1.

Fig. 3 is a set of curves showing the signal potentials at certain points in the circuit of Fig. 1.

Fig. 4 is a view of the oscillograph screen of the system, showing how the separate curves of two signals may appear thereon; and

Fig. 5 is a view of the oscillograph screen of the system showing how two signals may be added to produce a single curve thereon.

Referring first to Fig. l, the circuit therein disclosed comprises a first pair of input terminals ID to Whichone signal is to be applied, which terminals are connected between ground II and the cathode I2a of a vacuum tube l2. The terminals III are shunted by an input resistor IS. The tube [2 has a grid l2b which is connected by a series grid resistor It to a terminal I5 which is in turn connected to ground by a grid leak resistor 15. The terminal I5 is also connected through a resistor I! to a source of B plus potential and to one terminal of a condenser l8.

A second set of input terminals 20 is connected between ground and the cathode 22a. of a second vacuum tube 22. The grid 22b of this second tube is connected through a series grid resistor 24 to a terminal 25 which is also connected to ground by a resistor 26, to a source of B plus lpotential through a resistor 21, and to one terminal of a condenser 28. 1

The anodes I20 and 220 of the two tubes are connected to ground through a common load resistor 30, and the vertical deflection plates 3Ia of an oscilloscope 3| are connected in shunt to the resistor 30. i

The tubes 12 and 22 can be made conductive or non-conductive by the impression of suitable potentials upon the grids l2b or 22b. If both tubes are conductive then two signal currents applied to the input terminals l0 and 20 respectively will be added and their integrated result will be impressed upon the deflection plates 3la of the oscilloscope. On the other hand, if one tube is rendered non-conductive by the impression of a sum-- ciently high negative potential on its grid, and the other tube is made conductive by the impression of a suitable less negative or positive potential on its grid, then only the signal applied to the conductive tube will reach the deflection plates 3la of the oscilloscope.

In accordance with the invention, the signal currents impressed upon the input terminals I0 and 20 respectively are alternately applied to the deflection plates 310. by alternately rapidly changing the potentials of the grids [2b and 22b inversely with respect to each other. This is accomplished by alternately applying positive and negative potentials through the condenser 18' to the point 15 and simultaneously alternately applying negative and positive potentials through the condenser 28 to the point 25.

The desired positive and negative switching J potentials may readily be obtained from a multivibrator oscillator circuit. As shown in Fig. 1, such a circuit comprises a pair of vacuum tubes 35 and 36 having their cathodes 35a and 3601. connected to ground, having the anode 350 of tube 35 connected through a condenser 45 and a resistor- 31' to the; -grid,36b of tube 36, and the anode 36b of tube 36 connected through a condenser 38 and a resistor 39 to the grid 35b of tube 35. The resistors 40 and 40a provide the grid return to ground paths for tubes 35 and 36. The anodes 35c and 360 of the twotubjes-mayrbeenergized from a common source'o'f B plus patchtial through a switch 4|, a common resistor 42 and individual resistors. "s3. and. 44' re spectively. As is well known, whenthe switch M is closed, the multi-vibrator oscillatordisclosed will oscillate, at a frequency determined by the constants of the circuit, to produce substantially.

servedthatath-is- Wave is substantiall squaretopped or at least has substantially vertical sides of substantial magnitude. When point A in Fig. 1 goes positivethere is a tendency for current to flow directly from ground through the resistor I6, and indirectly from ground through the grid series resistor 14, the grid I21) and cathode 12a of -tube l2, and the resistor 3 to ground. The series-grid resistor H5 is of substantially lower value than the grid leak resistance It. Because of the relatively low resistance of the path including the resistor 14, the grid cathode path of the-tube l2 and the input resistor 13, considerable current flows to ground over this path in response to the application of positive pulses fromthe multi vibratcr to point A. On the other hand, when the potential at point A reverses and becomes negative, no current can flow through the resistor 14 and through the tube t2- and the resistor l3 to ground, because of the unidirectional conduction characteristics of the tube. Hencethe-only flow toground is throu hthe'grid leakresistor it (of relatively high value). l he result is that the potential variations at the point Bvary over a diiferent range than do the variations at point'A, and, as shown by curve B in Fig. 2-,, the potential at point B varies between a relatively low positive value and a-relative-ly high negative value. The positive half wave is sufficient to make the tube l2 conductive, whereas the negative half wave is sufficient to completely block the tube.

The. curve B is not fiat-topped because the con denser 18 charges and discharges. to a certain extent during each half wave. However, the positive halfwave applied to the grid lZb (point C) is as shown by curve C in Fig. 2,, substantially fiat-topped This is because of the fact that for positive pulses, the grid-cathode resistance ofthe thbe. l2" issmall relative .to the resistance of the series grid resistor It. In practice, the input resistor I3 may have a vameorcoo ohms,,the' series grid resistor M'may have 'a' value or 470,000 ohms, the grid leak resistor 18 may have a resistanceof l lllegbhfii, and the condenser I81 may have a capacity of 0.1 mfd. With these values, it is condenser 13.

found that if the wave A supplied by the multivibrator oscillator varies between plus 40 volts and plus 260 volts, the potential at point B, as shown by the curve B in Fig. 2, will decrease from an initial positive value of about 47 volts to a value of plus 3'7 volts and will then drop to a negative Value of minus 183 volts. "Became of the conduction characteristics of thetiibe, and the drop in the series grid resistor 54, the potential at the grid of the tube during the positive cycle, as shown by curve C in Fig. 2, will be approximately 1 volt positive. This is suflicient to make the tube-conductive, so that a D. C. potential applied between the terminals it will be conducted through the tube to the output resister 30 without distortion and without appreciable reduction in potential. During the other half cycle, the grid T21) of tube I2 is so negative with respect to the cathode I2a. that the tube is completely non-conducting. Hence potential on the input terminalsit will beapplied to the defleotionplates 31a-of the oscilloscope during, one half cycle, and completely interrupted during the next half cycle.

Signals are conducted through the tube l2 without amplification, because of the series grid resistor l4, which causes the grid to remain at substantially the same potential with respect to the cathode irrespective of the variationsin signal potential on thecathode. As an example, in the absence of a signal voltage, a potentialof approximately 1 volt positive (see curve C, Fig. 2) is maintained on the grid: 27). Now, if a signal of 2 volts negative is applied to the cathode, the charging current of the condenser it through vacuum tube [2 would increase due to the 2 volt signal. The increased grid current results in an increased drop across the series grid resistor It, therebycanoelling the difierence in potential between the grid and the cathode caused by the signal. When the signal is changed, the drop across the seriestresistor M changes accordingly, always-producing a degenerative or bucking effect. Hence noampli-fication takes place inthe tube because of the degenerative characteristics of the circuit, and apulsating unidirectional signal appliedto the input terminals it will be passed without destroying the D. C. reterence level thereof or any other characteristic of the signal.

The other half of the circuit, involving the input terminals 2-0, is identical with that half involving the-input terminals it, except that the condenser 28 has applied thereto a square-topped wave from the multi-vibrator oscillator that. is 180 degrees out of phase with thatapplied to the Hence, the tube 22 is alternately conductive and non-conductive to signals applied to the terminals 20 but its conductive and nonconductivep eriods are staggered with relation to those oi the tube i2",- so that signals from the input terminals ID and 2?! respectively are alterna'tely applied to the vertical deflection plates 3 la of the oscilloscope 3!. The result is that the electron beai'rl' in the oscilloscope-3i is alternately subjected to the signal potential on terminals l 0 and thatch terminals 20, with the result that twodiscon'tinuous curves are produced. By employing a multi vibrator that oscillates at 'a frequency' high relative to the time of persistence of vision, thesingle oscilloscope will provide two curves that appear to be continuous to the eye. This permits the'continuous displayo'f two independent curves representing two independently varying" functions on the screen are single userlo'sb'op e.

In Fig. 3 the combining of the two currents and their alternate switching is explained with short sections of graph showing the current amplified as to time. Curve D is the continuous curve representing variations in the magnitude of the unidirectional potential applied to terminals I0. Curve E is the discontinuous signal current passed by the tube [2 in response to this signal. Curve F shows the continuous or unidirectional signal applied to terminals 2 and curve G is the discontinuous signal current passed by the tube 22. It will be observed that the increments making up the curve E are staggered with respect to those making up curve G, and the currents represented by curves E and G will produce curves on the screen of the oscilloscope corresponding to curves H and I respectively. As shown in Fig. 3, curves H and I are clearly discontinuous, but if their frequency of interruption is high enough these curves H and I will appear continuous on the oscilloscope screen, as shown in Fig. 4.

It is sometimes desired to combine two signal currents, such as the signal currents applied to the input terminals l0 and respectively to produce a single curve corresponding to the sum of the two signal currents. A curve representing this summation can be produced with the apparatus of Fig. 1 by opening the switch t! to thereby cut off the B supply from the multivibrator and disable it. Under this condition, no A. C. impulses are applied through the condensers l8 and 28 to the switching circuit, and the grids l2?) and 22b assume a constant slightly positive value such as to render their associated tubes simultaneously conductive. This desired average potential is obtained by virtue of the connection of the terminals l5 and to the B-plus supply, through the resistors I1 and 27. In practice, when the resistors M, l6 and i3 have the values previously given, the resistors I! and 21 may each have a value of 2 megohms, which will result in a positive potential of approximately 1 volt on the grids of the tubes when no signal is applied thereto. However this voltage will remain substantially constant despite variations in the signal voltage, because of the degenerative effect of the circuit, as previously described.

Although for the purpose of explaining the invention, a particular embodiment thereof has been shown and described, obvious modifications will occur to a person skilled in the art, and I do not desire to be limited to the exact details shown and described.

I claim:

1. Apparatus for switching a unidirectional signal potential comprising: an electronic tube having a cathode, grid and anode; input and output resistors connected in series between said cathode and anode, the juncture of said resistors constituting a reference terminal; means for applying the said unidirectional signal potential across said input resistor, with its negative polarity connected to said cathode; a condenser, and a grid resistor connecting one terminal thereof to said grid; 2, source of periodically varying potential of substantially square wave form and means connecting it between the other terminal of said condenser and said reference terminal, the v-ari ations of potential of said source being of substantially larger order of magnitude than said signal potentials; and resistance means connect ing said one terminal of said condenser to said reference terminal, said grid resistor being of lower resistance than said last mentioned resistance means, and. both being large relative to said input resistor, and said condenser being sufliciently large relative to said resistance means and the frequency of said source to maintain a substantial charge during each half cycle of said source.

2. Apparatus according to claim 1 including a source of biasing potential having its negative terminal connected to said reference terminal, and a resistor of high value relative to said re sistance means connecting said other terminal of said condenser to the positive terminal of said source of biasing potential.

3. Apparatus for alternately applying unidirectional signal potentials from two separate sources to a single output circuit comprising first and second circuits each including: an electronic tube having a cathode, grid and anode, an input resistor connected between said cathode and a common terminal, a condenser, and a grid resistor connecting one terminal of said condenser to said grid, a grid leak resistor connecting said one terminal of said condenser to said common terminal, said grid resistor being of lower resistance than said grid leak resistor, and both being large relative to said input resistor; means connecting one of said sources of unidirectional signal current across the input resistor of said first circuit, and means connecting the other source of unidirectional signal current across the input resistor of said second circuit, each source having its negative terminal connected to the cathode of the associated tube; a source of periodically varying potential of substantially square wave form and having first and second output terminals supplying similar waves opposite in phase; means connecting said first output terminal to the other terminal of the condenser in said first circuit, and connecting said second output terminal to the other terminal of the condenser in said second circuit, said condensers being sufficiently large relative to said grid leak resistors and the frequency of said square wave source as to maintain a substantial charge during each half cycle of said square Wave; and means connecting the anodes of said tubes together and to said single output circuit.

DONALD W. BLANCHER.

Carnahan Nov. 10, 1942 Nicholson Sept. 30, 1947 Number 

